Method and installation for irradiating bulk material

ABSTRACT

The present invention is related to method for irradiating bulk material, particularly for treatment of food products such as grain, characterized in that in the case of a failure of said irradiation, the untreated bulk material is separated from the treated material and subsequently transported back towards the beginning of the process, in order to be treated. The invention allows to avoid re-treatment of large quantities of material, and provides a clear separation between contaminated and uncontaminated parts of the installation.

FIELD OF THE INVENTION

The present invention is related to a device for the irradiation of bulkproducts, in particular food products in bulk, such as grain.

STATE OF THE ART

The disinfestation of grain by irradiation is a known technique. Aradiation source, such as a beam of accelerated electrons or a beam ofX-rays is preferably used for this purpose. Many known techniques andinstallations, such as the one described in EP-A-705531 are related toirradiation of particles, preferably grain, as it falls through avertical shaft. When the irradiation is stopped during the process, forexample due to a machine failure, it is impossible to separate thetreated from the untreated grain. Even if the supply is stopped veryquickly, a minimum amount of untreated material will fall through theshaft without being irradiated, and join the already treated material,mixing with it. This causes large amounts of grain to be re-treated inorder to be sure that no untreated grain mixes with the already treatedbatch. Also, it requires parts of the installation which are normallyonly in contact with treated material, to be thoroughly cleaned anddisinfected after any machine failure. An additional drawback of thefree fall system is that the bulk material may be damaged in the fall.

Document EP-A-1080623 is related to a method for irradiating bulkmaterial, wherein a vertical conveyor system is used allowing the bulkmaterial to pass several times before the beam. The problem ofseparating untreated material from treated material in case of machinefailure is however not addressed in this document.

AIMS OF THE INVENTION

The present invention aims to provide a method and installation forirradiating bulk material, such as grain, an such a way that, in case offailure of the radiation source, one can separate untreated from treatedmaterial.

SUMMARY OF THE INVENTION

The present invention is related to a method for irradiating bulkmaterial, particularly for treatment of food products such as grain,comprising the steps of:

-   -   providing a continuous supply of said bulk material onto a        conveying device, said conveying device being at least partially        placed inside an irradiation cell, said conveying device being        exposed to an irradiation beam, allowing treatment of said        material,    -   evacuating the treated material at the end of said conveying        device,        characterised in that:    -   said conveying device transports said material in an essentially        horizontal direction, and    -   in the case of the loss or insufficiency of the irradiation, the        untreated and badly treated bulk material that should have been        treated properly, is separated from the fully treated bulk        material.

According to a preferred embodiment, said method further comprises anadditional recuperation step taking place in the case of insufficiencyor loss of the irradiation, said additional step consisting oftransporting said untreated bulk material, after being separated fromthe treated bulk material, towards the beginning of said conveyingdevice, in order for said untreated bulk material to be treated.

Said separation and recuperation are preferably obtained by performingthe following steps:

-   -   slowing down said conveying device at a predefined deceleration        rate,    -   blocking, inside said irradiation cell, the normal exit path of        irradiated material out of said irradiation cell,    -   creating, inside said irradiation cell, a new exit path, out of        said irradiation cell,    -   re-activating said conveying device, thereby forcing the        material that was present on said device to be evacuated from        the irradiation cell through said new exit path,    -   transporting said material that was present on said conveying        device back to the beginning of said conveying device.

Said step of creating a new exit path is preferably performed byrotating a hinged plate around an axis, over a predefined angle.

Said step or providing a continuous supply preferably comprises thefollowing substeps:

-   -   providing a continuous supply of said material to an input        hopper,    -   at a signal corresponding to an ‘empty’ condition in a weighing        station, transferring said material to said weighing station, by        opening a shutter underneath said input hopper,    -   at a signal corresponding to the ‘full’ condition of said        weighing station, closing said shutter underneath said input        hopper, weighing said material, and transferring said material        to a buffer hopper, by opening a shutter underneath said        weighing station,    -   transferring a continuous supply of said material from said        buffer hopper, onto said conveying device.

Said conveying device may be a conveyor belt and wherein a layer of aconstant width and thickness is preferably provided onto said conveyorbelt. Said layer or a constant width is preferably created by supplyingsaid material onto said belt by a slide, said slide being equipped withguides, said guides having preferably the shape of an inverse V. A layerof a constant thickness is preferably created by way of a horizontalscraper.

The invention is equally related to an installation for irradiating bulkmaterial, in particular for treatment of food products such as grain,comprising the following elements:

-   -   a device for creating a beam suitable for irradiation purposes,    -   an irradiation cell,    -   a supply system for providing a continuous supply of material to        said irradiation cell,    -   a first transport device for evacuating said material after        irradiation from said irradiation cell,        characterized in that said installation further comprises a        means for changing the exit path of said material after        irradiation from a first exit path, leading to said first        transport device to a second exit path.

The installation of the invention preferably further comprises a secondtransport device for transporting back towards said supply system, fromsaid second exit path, at least the untreated material present in saidirradiation cell, after a failure of said irradiation.

Said means for changing said exit path preferably comprises a hingedplate, operated by a cable, connected to a winch.

According to the preferred embodiment:

-   -   said supply means consist of a first hopper, above a weighing        station, comprising a second hopper and placed in turn above a        buffer hopper, leading to a screw conveyor, which takes the        material through the wall of the irradiation cell,    -   a conveyor belt is placed wholly inside said irradiation cell,        said conveyor belt comprising vertical ridges perpendicular to        the translation of said conveyor belt,    -   a slide is placed between said screw conveyor) and said conveyor        belt, said slide being equipped with guides, said guides having        the shape of an inverse V,    -   said first and second transport devices are both screw        conveyors, said installation further comprising two material        elevators for transporting the material after said first and        second screw conveyors, said second material elevator        transporting the material back to said buffer hopper in case of        an irradiation failure.

According to a second embodiment:

-   -   said supply means consist of a first hopper, above a weighing        station, comprising a second hopper and placed in turn above a        buffer hopper, leading to a screw conveyor, which takes the        material through the wall of the irradiation cell,    -   said screw conveyor transports said material underneath said        beam for irradiation,    -   said first and second transport devices are both screw        conveyors, said installation further comprising two material        elevators for transporting the material after said first and        second screw conveyors, said second material elevator        transporting the material back to said buffer hopper in case of        an irradiation failure.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a schematic view of an installation and of the processflow according to the present invention.

FIG. 2 represents a detailed view of an installation according to apreferred embodiment of the present invention.

FIG. 3 represents a detail of the conveyor belt used during irradiation,according to a preferred embodiment of the present invention.

FIG. 4 represents a detail of the slide used in the installationaccording to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic view of the essentials in the process flowaccording to the method of the invention. The flow has to be clearlydivided into two parts: untreated 100 and treated material 101. Thetransition from untreated to treated material is made in the irradiationcell 3.

According to a preferred embodiment, this material consists of grain, tobe irradiated for disinfestation purposes. It may however equallyconsist of other food products such as fruit or potatoes, or of polymerpowder or pellets. The treatment of the latter may comprise a graftingor curing operation.

According to the preferred embodiment of the method of the invention,the untreated material is brought by conveyors from the material storage(not shown) into a buffer hopper 1. A bulk material transport device 2,which is preferably a screw conveyor, feeds a continuous stream of bulkmaterial through the wall of the irradiation cell 3 and onto a slide 4,allowing the bulk material to slide onto a conveying device 5, which ispreferably a horizontal conveyor belt. The belt is preferably made ofstainless steel jointed slats, or stainless steel webbing. The conveyingdevice may also be an “en-masse” conveyor, wherein the bulk material istransported in a u-shaped trough by horizontal slats driven by chains.Special guides are present on the slide 4, in order to distribute thebulk material in a layer which covers the whole width of the conveyorbelt 5. A blade system on the slide 4 will make sure that the bulkmaterial layer thickness is equal to a predefined value over the wholewidth of the conveyor belt 5. These features are explained in moredetail further in this description. Once the bulk material is on theconveyor belt 5, it will be transported under the beam 6 and irradiatedby this beam, which may be produced by an electron accelerator (notshown).

According to a second embodiment of the invention, the screw conveyor 2,slide 4 and conveyor belt 5 are replaced by one screw conveyor, and thebulk material is irradiated while being transported by that one screwconveyor, underneath the beam. The preferred embodiment howevercomprises a screw conveyor 2, slide 4 and a horizontal conveyor belt 5,carrying a layer of bulk material of a constant thickness underneath thebeam 6.

The irradiation of the bulk material marks the transition from theuntreated part 100 of the process to the treated part 101. Aftertreatment, the bulk material drops into a funnel, and is normally ledinto a transport device 7, preferably a second screw conveyor, whichtransports the bulk material out of the irradiation cell, through thewall of the cell, and feeds it into a bulk material elevator 8. Fromhere the bulk material is transported to the treated bulk materialstorage.

If a failure occurs during the irradiation process, the conveyor belt 5and the supply screw conveyor 2 are immediately stopped. Such a failuremay be a complete loss of irradiation, or an insufficient irradiationdue to a defect in the radiation source. Once the belt 5 is stopped, thematerial that is still on it is either badly or insufficiently treatedor not treated at all. Means 40 are provided to lead the untreated andbadly treated bulk material that should have been treated properly, notinto the evacuation screw conveyor 7, but into a transport device 11,preferably a third screw conveyor, which recuperates the untreated andbadly treated bulk material and sends it back to the start of theprocess flow, in order to be re-treated. The untreated and badly treatedbulk material is evacuated through the screw conveyor 11 and fed into abulk material elevator 12. On top of the bulk material elevator 12 thebulk material is released into the buffer hopper 1 in order to be(re)treated.

The invention is characterised by the presence of the means 40 forseparating untreated and badly treated bulk material in case of anirradiation failure. This feature avoids any mixing of untreated andtreated materials. Also, a clear separation between untreated andtreated parts of the installation is present, allowing a quick restartin case of failure, obviating the necessity for a thorough cleaning ordisinfecting operation. In a preferred embodiment, the invention alsocomprises means for recuperating and recirculating (11,12) untreated andbadly treated bulk material. In the following, a preferred embodiment ofthe method and installation, including the means 40 for separating thematerials is described in detail.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 2 shows a detailed view of an installation according to a preferredembodiment of the invention. FIG. 2 b and 2 c show a frontal and sideview of the whole of the installation, FIG. 2 a shows a side view of thesupply system only. Many of the elements described above can berecognised: irradiation cell 3, buffer hopper 1, supply screw conveyor2, slide 4, conveyor belt 5, evacuation screw conveyor 7, recuperationscrew conveyor 11, evacuation elevator 8, recuperation elevator 12. Thebeam is produced by an electron accelerator 20.

In order to provide a continuous supply of bulk material into theirradiation cell, an input hopper 21 and a semi-continuous weighingstation 22, comprising a weighing hopper 23 are present. The function ofthe input hopper 21 before the semi-continuous weighing station 22 is toallow a continuous bulk material flow coming from the untreated bulkmaterial storage and to stop the bulk material flow when thesemi-continuous weighing station 22 is weighing. Once the weighingstation is full, a shutter 24 is closing the supply from the inputhopper 21 into the weighing station 22. Once the bulk material has beenweighed, the shutter 25 of the weighing station opens and allows theweighing station's hopper 23 to be rapidly emptied into the bufferhopper 1. When the weighing station is empty, its shutter 25 is closedand the shutter 24 of the input hopper is opened again.

Being placed after the weighing station 22, the buffer hopper 1 absorbsthe discontinuities in the bulk material flow due to said weighingstation 22. The irradiation process requires a continuous bulk materialsupply in order to obtain acceptable dose homogeneity and sufficientcontinuity of the process. The buffer hopper 1 also serves to acceptadditional bulk material volumes coming from the recycling of badlytreated or untreated bulk material (via the bulk material elevator 12).The process control system will ensure that the buffer hopper 1 does notget refilled with the bulk material to be recycled or with bulk materialcoming from the weighing station 22 unless its level is low enough toallow this.

From the buffer hopper 1, the bulk material is allowed through a shutter26 into the screw feeder conveyor 2. This type of screw conveyor ispreferably designed to be flood loaded and to regulate the volume ofmaterial being delivered at its output.

The bulk material, which is delivered in a controlled way by the screwfeeder 2, drops into a funnel 30 at the top of the slide 4. FIG. 3 showsan enlarged image of the installation inside the irradiation cell. Theslide 4 is constructed in such a way as to fulfil the followingrequirements:

-   -   At the outlet of the slide (bottom part) the bulk material flow        is homogenous over the entire width of the conveyor belt 5: this        is obtained by placing guides on the path of the sliding bulk        material. These guides may be a set of chevrons 34 (inverse V)        spread over the surface of the slide as shown on FIG. 4.    -   The layer thickness on the conveyor belt is constant: this is        obtained by placing an adjustable blade horizontally at the        outlet end of the slide and at a fixed distance above the        conveyor belt surface.

According to the preferred embodiment, the width of the belt 5 is 2meters, and the thickness of the layer is controlled at 2.4 cm. With abelt speed of 44 meters per minute, this allows a bulk materialthroughput of 100 tonne per hour, considering the bulk material densityto be in the range between 0.75 and 0.8 tonne/m³. Using an electronaccelerator such as a Rhodotron or a Dynamitron, having a beam energy of5 Mev, and a power of 200 kW, would allow treatment of such a quantity.The thickness of 2, 4 cm for a material having a density between 0,75and 0, 8 tonne/m3 is chosen such that is penetrated by a 5 MeV electronbeam in such a manner that the dose deposited in the upper layers ofmaterial is approximately equal to the dose deposited in the lowerlayers. Assuming that 55% of the power is actually deposited in thegrain, a 200 kW beam can achieve a 4 kGy dose in a 100 tonne/hourthroughput. A dose of 4 kGy is known to be enough to kill TCK (TilletiaControversia Kuhn) in grain.

The bulk material is now homogeneously spread over the entire width ofthe conveyor belt 5 in a continuous layer. Vertical ridges 31 on theconveyor belt, assure that the bulk material is not sliding on thebelt's surface when it first falls on the belt and needs to beaccelerated. The cross section of these vertical ridges 31 may be arectangle mounted perpendicularly to the conveyor's surface and to theconveyor's translation. However, such vertical ridges would impose apeak loading on the conveyor system each time such a ridge passes underan adjustable blade. In order to prevent such peak loading, the verticalridges are preferably given an inverse V-shape or a curved shape.

The bulk material is now transported through the accelerated electronbeam. When no problems occur during the irradiation process the bulkmaterial drops off the conveyor at the end and into a funnel 32. This isthe funnel that feeds treated bulk material into the treated bulkmaterial screw conveyor 7.

If a problem occurs during the irradiation process, the conveyor 5 isbrought to a complete stop by gradually decelerating. Also, the screwfeeder 2 is stopped immediately. The conveyor belt's 5 length has beencalculated in order to be able to stop the belt completely before any ofthe untreated bulk material falls off at its end. The deceleration is tobe calculated in order to assure that none of the bulk material of theconveyor is slung over the top of the vertical ridges 31 during thedeceleration. This assures that no untreated and badly treated bulkmaterial falls off the conveyor belt 5 and into the treated bulkmaterial funnel 32. The belt being stopped, it is now needed to recyclethe untreated and badly treated bulk material which is still on the belt5 and on the slide 4, to the input of the supply system. According tothe preferred embodiment shown in FIG. 3, a hinged plate serving as achute 40 is put into place, by traction on its command cable. Thecommand cable is pulled on by an electric winch, which is placed in oneof the product pits, outside the irradiation cell. This chute 40 allowsto empty the conveyor into the second funnel 33 the untreated bulkmaterial funnel. The conveyor belt is restarted at a low speed (e.g. 2m/min) and with the chute 40 in place, the bulk material now falls intothe untreated bulk material funnel 33, where it is fed into theuntreated bulk material screw conveyor 11. The conveyor 11 runs untilthe belt 5 as well as the slide 4 at the input side are empty, and thebulk material that was present on it is evacuated by the elevator 12,which leads it back to the buffer hopper 1. Once this is done, the chute40 is put back to its normal position, to allow the bulk material tofall once again into the treated bulk material funnel 32, and normalproduction can be restarted.

Both screw conveyors 7,11 for extracting the treated and the untreatedand badly treated bulk material out of the irradiation cell 3 areregular screw conveyors. The throughput of the untreated bulk materialscrew conveyor 11 can be much lower than the one for the treated bulkmaterial 7, since in normal operation all the bulk material is treated.According to the preferred embodiment, the screw conveyor 7 for thetreated bulk material will have a throughput of 100 tonne per hour. Theoutput side of both screw conveyors 7 and 11 is at the outside of theirradiation cell in so-called product pits (51 and 50 respectively).

The exact configuration of the bulk material elevators for the treatedbulk material 8 and the untreated bulk material 12 will strongly dependupon the configuration of the site. The bulk material elevator 8 for thetreated bulk material will need to feed into a conveying system thattransports the bulk material to the treated bulk material storage. Theuntreated bulk material screw conveyor 11 may either feed into a bulkmaterial elevator 12 for automatic recycling of the untreated bulkmaterial or into a bin. This bin, when full, will then need to bemanually emptied into the buffer hopper 1. To do so a lifting devicewill be needed, e.g. a crane.

In order to avoid throughput variations due to an irregular bulkmaterial feed flow, it is necessary to control the product supply system(21,22,1,2) through a control system. The control system will take careof flow management, e.g.: do not empty the weighing station 22 into thebuffer hopper 1 if it is filled with bulk material above a determinedlevel. The control system will also take care of the management ofaccelerator failure, by stopping the belt 5 and supply screw conveyor 2.

In order to perform the control tasks, a number of sensors are needed.These are primarily level measurement sensors in the hoppers but thecontrol system also analyses equipment status information (conveyorspeed, conveyor running signal, . . . ) and radiation source information(beam intensity, beam OK signal, . . . ).

In order to avoid damage by irradiation, no sensitive parts should beplaced in the irradiation cell 3, in particular sensors which containsemiconductors or parts which are manufactured in radiation sensitivematerial. According to the preferred embodiment, all this equipment istherefore placed outside the irradiation room.

The safety of the entire facility is controlled by a dedicated securitycontrol system. The security system controls access to the irradiationcell in respect to the status of the accelerator and the handlingequipment. Particular attention is given to the access to the productpits (50,51).

When the accelerated electrons collide with the air particles, ozone isgenerated. Another task of the safety system is the control over theozone extraction system, and the management of access to the irradiationcell in respect with ozone concentration.

1. An apparatus which irradiates bulk material, the apparatuscomprising: a source which produces an irradiation beam for sterilizingthe bulk material, an irradiation cell, a supply system for providing acontinuous supply of material to the irradiation cell, wherein thesupply system comprises a conveying device configured for moving thebulk material in a downstream direction to both evacuate irradiatedmaterial in a first exit path and redirect inadequately irradiatedmaterial in a second exit path, a first transport device for evacuatingthe material after irradiation from the irradiation cell in the firstexit path, and a device configured to redirect inadequately irradiatedmaterial after the irradiation cell from the first exit path to thesecond exit path in case of inadequate irradiation.
 2. The apparatusaccording to claim 1, further comprising a second transport device fortransporting the inadequately irradiated material back towards thesupply system from the second exit path after an irradiation failure. 3.The apparatus according to claim 1, wherein the device configured tochange the exit path comprises a hinged plate, operated by a cable,connected to a winch.
 4. The apparatus according to claim 2, wherein:the conveying device comprises a screw conveyor; the supply systemcomprises a first hopper above a weighing station which includes asecond hopper arranged above a buffer hopper leading to the screwconveyor configured to transport the material through the wall of theirradiation cell, a conveyor belt having a direction of translationarranged inside the irradiation cell the conveyor belt comprisingvertical ridges perpendicular to the direction of translation of theconveyor belt, a slide arranged between the screw conveyor and theconveyor belt, the slide having guides with the shape of an inverse V,the first and second transport devices are screw conveyors, and theapparatus further comprising two material elevators for transporting thematerial after the first and second transport devices, the secondmaterial elevator configured to transport the material back to thebuffer hopper in case of an irradiation failure.
 5. The apparatusaccording to claim 2, wherein: the conveying device comprises a screwconveyor; the supply system comprises a first hopper, above a weighingstation, comprising a second hopper arranged above a buffer hopperleading to the screw conveyor configured to transport the materialthrough the wall of the irradiation cell and underneath the beam forirradiation, the first and second transport devices are screw conveyors,and the apparatus further comprises two material elevators fortransporting the material after the first and second transport devicesthe second material elevator configured to transport the material backto the buffer hopper in case of an irradiation failure.
 6. An apparatuswhich irradiates bulk material, the apparatus comprising: a source whichproduces an irradiation beam for sterilizing the bulk material, anirradiation cell, a supply system for providing a continuous supply ofmaterial to the irradiation cell, a first transport device forevacuating the material after irradiation from the irradiation cell, adevice configured to change the exit path of the material afterirradiation from a first exit path leading to the first transport deviceto a second exit path in case of irradiation failure, and a secondtransport device for transporting untreated material in the irradiationcell back towards the supply system from the second exit path after anirradiation failure.
 7. The apparatus of claim 6, wherein the supplysystem comprises a first hopper above a weighing station which includesa second hopper arranged above a buffer hopper leading to a screwconveyor configured to transport the material through the wall of theirradiation cell, wherein the first and second transport devices arescrew conveyors, and wherein the apparatus further comprises: a conveyorbelt having a direction of translation arranged inside the irradiationcell, the conveyor belt comprising vertical ridges perpendicular to thedirection of translation of the conveyor belt, a slide arranged betweenthe screw conveyor and the conveyor belt, the slide having guides withthe shape of an inverse V, and two material elevators for transportingthe material after the first and second transport devices, the secondmaterial elevator configured to transport the material back to thebuffer hopper in case of an irradiation failure.
 8. The apparatus ofclaim 6, wherein the supply system comprises a first hopper, above aweighing station comprising a second hopper arranged above a bufferhopper leading to a screw conveyor configured to transport the materialthrough the wall of the irradiation cell and underneath the beam forirradiation, wherein the first and second transport devices are screwconveyors, and wherein the apparatus further comprises two materialelevators for transporting the material after the first and secondtransport devices, the second material elevator configured to transportthe material back to the buffer hopper in case of an irradiationfailure.